Four-switch non-inverting buck-boost switching converters can switch between different modes of operation depending on the relation between an input voltage and an output voltage of the switching converter. FIG. 1 is a schematic of a four-switch non-inverting buck-boost switching converter 10. An input voltage VIN is received at an input terminal and an output voltage VOUT is provided at an output terminal. An inductor 12 has a first terminal coupled to a first set of switches 14 at a first node LXU and a second terminal coupled to a second set of switches 16 at a second node LXO. An error amplifier 18 is used to generate a control signal based on the output voltage VOUT and a reference voltage VREF. The control signal is provided to a controller 19, and the controller 19 provides the control signal to regulate the power switches 14, 16. The first set of power switches comprises a high side switch that couples the inductor 12 to the input voltage VIN when it is in a closed state, and the second set of power switches comprises a high side switch that couples the inductor 12 to the output voltage VOUT when it is in a closed state.
If the input voltage VIN is substantially greater than the output voltage VOUT then the switching converter can operate in a buck mode by only operating the first set of switches 14, which may be referred to as the buck mode power switches. In the buck mode, the high side switch of the second set of switches 16 is in the closed state and couples the inductor 12 to the output voltage VOUT. If the input voltage VIN is substantially less than the output voltage VOUT then the switching converter can operate in a boost mode by only operating the second set of switches 16, which may be referred to as the boost mode power switches. In the boost mode, the high side switch of the first set of power 14 switches is in the closed state and couples the inductor 12 to the input voltage VIN. If the input voltage VIN is approximately equal to the output voltage VOUT then the switching converter operates in a buck-boost mode, which may be referred to as a buck-boost window, where all power switches 14, 16 are used to regulate the output voltage VOUT.
There are several existing methods of controlling the four-switch non-inverting buck-boost switching converter and selecting the appropriate operation mode. FIG. 2 is a schematic of a first implementation of a four-switch non-inverting buck-boost switching converter 20. The output voltage VOUT and the reference voltage VREF are received at an error amplifier, and an error amplifier voltage VEA is provided to a first comparator 22 and a second comparator 24. The first comparator 22 receives a first voltage ramp and the second comparator 24 receives a second voltage ramp. An output of the first comparator 22 is provided to a reset terminal of a first SR latch 26. An output of the second comparator 24 is provided to a reset terminal of a second SR latch 28. The first SR latch 26 receives a first clock signal and the second SR latch 28 receives a second clock signal. The first SR latch 26 provides the control signal MagBOOST to operate the second set of switches 16 and the second SR latch 28 provides the control signal MagBUCK to operate the first set of switches 14.
The method of controlling the four-switching non-inverting buck boost switching converter 20 is typically referred to as “voltage mode”. This is in reference to the comparison of voltages, namely the voltage ramps with the error amplifier voltage VEA.
FIG. 3 is a plot of showing the operation of the switching converter 20 where the input voltage VIN decreases from left to right. The decreasing input voltage VIN results in an increasing error amplifier voltage VEA. FIG. 3 shows a second clock signal 30, a first clock signal 31, a first voltage ramp 32, a second voltage ramp 33, an error amplifier voltage VEA 34, a control signal MagBUCK 35, a control signal MagBOOST 36, and a current 37 through the inductor 12.
For a small output voltage VOUT compared to the input voltage VIN, the error amplifier voltage VEA is small such that the error amplifier voltage VEA intercepts the second voltage ramp 33 such that the control signal MagBUCK 35 provides a suitable signal for regulation of the first set of switches thereby operating the switching converter 20 as a buck converter.
For a large output voltage VOUT compared to the input voltage VIN, the error amplifier voltage VEA is large such that the error amplifier voltage VEA intercepts the first voltage ramp 32 such that the control signal MagBOOST 36 provides a suitable signal for regulation of the second set of switches thereby operating the switching converter 20 as a boost converter.
For an output voltage VOUT approximately equal to the input voltage VIN, the error amplifier voltage VEA intercepts the first voltage ramp 32 and the second voltage ramp 33, such that the control signal MagBUCK 35 and the control signal MagBOOST 36 provide suitable signals for regulation of the first set of power switches and the second set of switches thereby operating the switching converter 20 as a buck-boost converter.
A schematic of a second implementation of a four-switch non-inverting buck-boost switching converter is shown in FIG. 4. The four-switch non-inverting buck-boost switching converter of FIG. 4 is a current mode buck-boost converter 40. Operation of the current mode buck-boost converter 40 comprises three phases: a first phase, relating to the boost mode operation and a charging current; a second phase, relating to the buck mode operation and a discharging current; and a bypass phase where a first high side switch 42 and a second high side switch 44 are closed. The buck-boost converter 40 comprises a current sensing function.
The current mode buck-boost converter 40 comprises a current sensing resistor 46, which has a resistance of 10 mΩ. The current sensing resistor 46 converts current flowing through an inductor 48 into a sensed voltage. The sensed voltage is received by a first comparator 41 and a second comparator 43. The sensed voltage is compared with a first constraint −k·VEA by the first comparator 41 and the sensed voltage is compared with a second constraint +k·VEA by the second comparator 43. An output of the first comparator 41 is used to generate the control signal MagBUCK and an output of the second comparator 43 is used to generate the control signal MagBOOST. The method of controlling the current mode buck boost switching converter 40 is typically referred to as “current mode”.
The switching converters shown in FIGS. 1, 2 and 4 require a constant clock. Additionally, the switching converters shown in FIGS. 1, 2 and 4 rely on process and timing dependent comparators and voltage ramps, which can result in variation in the voltages at which the error amplifier voltage VEA overlaps with the voltage ramps 32, 33. This can change the input voltage VIN and output voltage VOUT values over which the switching converters operate in the buck-boost mode. Trimming steps or extra correcting loops are required to resolve this issue.
The switching converters of FIGS. 1, 2 and 4 also require the design of extra analog circuit blocks, including: oscillators, current sensors, comparators and compensation. Therefore, it is desirable to reduce complexity of the above switching converters.